Verantw. Gesuchsteller/in und weitere Gesuchstellende

Abstract

The upwelling and subduction of water masses in the Southern Ocean plays a vital role in the global climate system, because they redistribute carbon and heat between the atmosphere and the deep ocean. Human-induced future changes in the marginally stable vertical density gradient (here referred to as stratification) of this region could critically alter this vertical exchange and therefore the uptake and release of carbon-dioxide (CO2) and heat by the ocean. Therefore, such changes could considerably diminish or amplify global warming. Yet, current global climate models reveal large biases in this region leading to large uncertainties in future climate projections. In this project, we investigate the effect of stratification changes on the future CO2 and heat uptake by the Southern Ocean and constrain the related uncertainty in projected future global climate change. For this purpose, we hypothesize that the simulated surface freshwater flux balance is a key driver of the Southern Ocean stratification in global climate models and determines their sensitivity of the future uptake of CO2 and heat to climatic changes. We will approach this challenge in two ways: On the one hand, we will use new observational constraints in surface freshwater, heat, and CO2 fluxes to evaluate the existing future projections. This analysis will provide insights into the process responsible for the large discrepancies between models and help to constrain uncertainties. On the other hand, we will tweak an existing global climate model to provide a more realistic Southern Ocean circulation by adjusting the surface freshwater fluxes using observational estimates. We will then impose freshwater flux scenarios by perturbing the surface fluxes according to expected future changes to study the response of CO2 and heat uptake to changes in the surface freshwater fluxes. This latter analysis might also reveal potential surprises in the effect of Southern Ocean stratification changes on global warming that might not be represented in current global climate models due to their biases.